1. Field of the Invention
The present invention relates to a signal recording method for an optical disk and an apparatus therefor, and more particularly it relates to a signal recording method by a mark length recording system for erasable type optical disk and to an apparatus therefor.
2. Description of Related Art
In an optical disk drive unit in which an erasable type optical disk is used as a recording data medium, where a signal being outputted from an EFM (Eight to Fourteen Modulation) encoder is directly modulated by an optical modulation system and its mark length is recorded in the optical disk, and when its recording linear velocity becomes lower, a recorded bit is not precisely formed on the optical disk, then, there occurs such an apprehension as that sufficient reproduced waveforms can not be obtained. In order to avoid such apprehension, it is necessary to keep the recording linear velocity to be more than a predetermined value. There, then, still exists another apprehension that the value of the minimum limit of the recording linear velocity becomes so high that an area in which the recording linear velocity is usable becomes narrower, then, the maximum time for recording on the optical disk becomes shorter.
In order to avoid such another apprehension, as shown in "High-Density Magneto-Optic Disk Using Highly Controlled Pit-Edge Recording" (Proc. Int. Symp. on Optical Memory, 1987 Japanese Journal of Applied Physics. Vol. 26 '87 Supplement 26-4), there is proposed a method of adjusting a distribution of intensity of a laser power upon signal pulse by operating its circuit. By this method, however, the system construction becomes more complicated and sufficient characteristics for recording signal can not be obtained in the area of lower linear velocity.
FIG. 1(a), for example, shows a binary signal (signal to which EFM is applied) to be recorded on a magneto-optic disk. Laser beam is irradiated in response to a length of the value "1" of this signal and a laser irradiation bit is formed on the magneto-optic disk, and laser is turned off in response to the length of the value "0" of this signal and a laser turn-off bit is formed on the magneto-optic disk, respectively. FIG. 1(b) shows a laser output power in each of those cases being mentioned above. And in FIG. 1(c), the protruding portion shows the laser irradiation bit which is formed on the optical disk in response to the signal "1", and the recessed portion shows the laser turn-off bit which is formed on the optical disk in response to the signal "0", respectively.
As shown in FIG. 1(c), the laser irradiation bit grows to have more length than the input signal "1" because of thermal diffusion which is caused when the laser is irradiated on the magneto-disk, and the growth of the laser irradiation bit erodes and shortens the laser turn-off bit.
Because of such phenomenon as mentioned above, the laser irradiation bit will largely erode the laser turn-off bit in the area of lower linear velocity, then, sufficient characteristics can not be obtained when a reproduced signal is obtained.